The Food and Drug Administration requested Wednesday that manufacturers immediately pull all prescription and over-the-counter versions of the drug.

The trouble with ranitidine, the active ingredient in Zantac and its equivalents, rests with a contaminant known as N-Nitrosodimethylamine, or NDMA. The agency says that over time, NDMA — a probable cancer-causing substance that’s also found in various foods, including cured meats — shows up as an impurity in ranitidine in concentrations that exceed federal standards.

The FDA issued a warning last September about the drugs, prompting pharmacies, including CVS, Walgreens and Walmart, to halt sales of Zantac and its equivalents. Now, amid an ongoing investigation, the FDA says it has confirmed that the issue is linked with the way ranitidine naturally breaks down over time in normal storage conditions — not how it is manufactured.